THE LEAF 109 



which enters the plant is broken up within the cell, and a 

 portion of the oxygen is given off. This taking away and 

 liberation of oxygen from the carbonic acid is termed a process 

 of reduction. At the same time a very complicated process 

 of transformation of organic substances is going on (metabolism), 

 during which oxygen is taken up and carbonic acid is given 

 off, which shows that a process of oxidation is also going on. 

 The giving off of carbonic acid, due to the oxidisation of the 

 organic substances (combustion of substance), is of the same 

 nature as the breathing in of oxygen by animals, and the exha- 

 lation of carbonic acid, and is therefore termed respiration 

 also in the case of vegetable tissues. This process takes place 

 in darkness as well as in the light. 



All parenchyma is not meant for assimilatory processes. 

 We have already studied the parenchymatous cells of the pith, 

 which rarely contain chlorophyll, and have a special function 

 to perform. In other cases, the parenchymatous cells may 

 have the function of a supporting or strengthening tissue. In 

 such a case, the cell-walls become thickened in the corners 

 where three or more cells meet, and the chlorophyll granules 

 are very slightly developed. Such a tissue receives the special 

 name of eollenehyma. 



The amount of assimilation which takes place depends upon 

 the number of chlorophyll granules present in the cell. For 

 it is these bodies which make use of energy latent in the rays 

 of light, and perform the actual assimilation. This chlorophyll 

 occurs in the phanerogamic plants in the form of granules or 

 corpuscles, while in some of the algas it assumes the form of 

 plates or bands. The form of granule seems the most prac- 

 tical, for it offers the greatest amount of surface for the smallest 

 amount of substance. In this way, within the smallest space 

 the largest amount of light is absorbed, and "light is life." 

 The endeavour to absorb the greatest possible amount of light, 

 which is apparent in most organisms, is made evident in plants 

 by the phenomenon of positive heliotropism, i.e., the faculty of 

 placing their organs in such a position with reference to the 

 source of light, that the incident rays will be most advan- 

 tageously absorbed. Bach chlorophyll granule consists of a 

 protoplasmic framework, of a spongy nature, which is itself 



